1. Field of the Invention
The present invention relates generally to battery charging and, more specifically, to a DC-DC converter and battery charger, which uses a direct oxidation fuel cell as a power source for charging a re-chargeable battery.
2. Background Information
There are numerous conventional techniques and components (e.g., off-the-shelf integrated circuits) for charging re-chargeable batteries such as lithium-ion batteries widely used in consumer electronic devices. Typically, an AC wall outlet or a 12 V DC source, commonly provided in automobiles, is used as a power source for this type of recharger.
One disadvantage of such conventional re-chargers is that they may not be particularly efficient, meaning that the re-chargers do not transfer a high percentage of power from the power source to the battery over a range of expected operating conditions. This is not surprising because high efficiency is not generally a criterion of excellence for such re-chargers. Rather, the ability to rapidly re-charge the battery and maintain or extend the battery""s useful life is considered very important.
However, if a fuel cell, such as a direct oxidation fuel cell, is used as the power source, then the re-charger""s efficiency becomes a much more important consideration. First, a fuel cell, which is part of a portable device, like a battery re-charger, has a finite amount of fuel available to it. In general, available fuel should be carefully managed to maximize user convenience, maximize the operating time of whatever device the fuel may power, and extend the time between re-fuelings. Thus, a highly efficient re-charger is desirable, if not essential, for realizing the substantial advantages of using a fuel cell as a power source for the re-charger.
In brief summary, the present invention provides a method and apparatus for actively controlling the operating point, i.e. the output voltage or current, of a direct oxidation fuel cell or fuel cell stack and enables efficient transfer of power from the fuel cell to a re-chargeable battery and load.
In accordance with one aspect of a preferred embodiment of the present invention, a DC-DC converter is coupled between a direct oxidation fuel cell and a parallel combination of a re-chargeable battery and load. In this arrangement, the output voltage of the fuel cell is supplied as the input voltage to the DC-DC converter. The output of the converter is preferably connected directly to the battery/load combination. As a result, the output voltage of the DC-DC converter equals the battery voltage, and the converter behaves as an unregulated current source whose output current either charges the battery or helps the battery supply current to the load.
The output voltage of the fuel cell is advantageously used as a closed loop feedback signal to control the duty cycle of the DC-DC converter switch elements. The feedback signal is compared with a reference that represents a predetermined, optimum output voltage for the fuel cell. Alternatively, it is possible to adjust the reference voltage to optimize the fuel cell output under different operating conditions. The closed loop operates to reduce to zero the difference between the feedback signal and reference. As a result, the fuel cell""s output voltage is kept substantially constant over a wide range of battery voltages. In addition, while variations in the fuel cell""s operating conditions (e.g., temperature, fuel flow rate and the like) will cause corresponding changes in the output current of the fuel cell, the fuel cell""s output voltage is maintained generally constant by the device of the present invention.
Another advantage of the present invention is that by providing effective control over the operating voltage of a fuel cell stack, it is possible to maintain a safe minimum voltage which will prevent any cell in the stack from being reversed and possibly damaged due to a high load current and insufficient reactants in the cell. Similarly, effective control over the operating voltage enables different operating voltages to be established for different fuel concentrations, an important factor in attaining maximum efficiency from a direct methanol fuel cell.
In accordance with another aspect of a preferred embodiment of the present invention, a shunt voltage regulator is placed in parallel with the battery/load combination to protect the battery from an over-voltage condition.
In accordance with an alternative embodiment of the present invention, rather than using the fuel cell""s output voltage as the feedback signal, the fuel cell""s output current may be used. In such an arrangement, it is the fuel cell""s output current, which is maintained essentially constant while the output voltage may vary.
Experiments have shown that using the present invention over a range of fuel cell output voltages, which are typical for direct methanol fuel cells, and a range of battery voltages typical for lithium-ion batteries, power efficiencies in excess of 90% are achievable.